see this video how to change the TWRP language if you don't understand Russian and downloaded a default Russian version
https://www.youtube.com/watch?v=vxPeYJYsUYU
No is a good time to insert a micro SD card and backup all partitions in TWRP, especially the nvram

Phone is always rebooting into TWRP Recovery, no matter what reboot option you choose.
therefore we try to lock the bootloader again

reboot to bootloader, at the same time run
fastboot oem lock
question on the phone
lock bootloader?
confirm with volume_up
fastboot oem lock
< waiting for device >
...
(bootloader) Start lock flow

OKAY [ 7.943s]
finished. total time: 7.943s

10 seconds power button to switch off
reboot system
no warning State Orange

(
Wipe
Dalvik / ART Cache
Cache
)notwendig?

flash supersu from SD card
reboot

Where to find the OTA update file in Android 7?
here it was downloaded to /data/data/com.adups.fota/files/adupsfota/update.zip

How to uninstall superSU?
https://forum.xda-developers.com/showpost.php?p=63615067&postcount=2459

K9 mail stops syncing after some time, as caldav seems to do.
It seems to help to deactivate battery optimisation
go to Settings - Battery - menu: battery optimization
select 'All apps'
tap on K-9 Mail, select 'Don't optimize'
confirm
now syncing should work all the time

THL 5000

rooting

Rooting works with baidu root, version 2.4.6 (at the moment available only in
Chinese). I tried framaroot and towelroot, none of them worked.
The root is permanent and stays after a reboot.
After rooting install SuperSU instead of the Chinese root Permission program.
Then you can remove all the Chinese programs (one of them displays Chinese ads
in the status bar).

The only solution that worked for me was installing the recovery.img by hand on
the phone itself. But a recent custom recovery is needed to install the custom
ROM, wipe the dalvik cache and other things.

# fastboot update Cyanogen-VX.zip
archive does not contain 'android-info.txt'
archive does not contain 'android-product.txt'
error: update package has no android-info.txt or android-product.txt

reboot to bootloader
adb reboot bootloader

when the phone has booted into fastboot unlock the bootloader (I don't know if
this is really necessary, please give feedback)fastboot oem unlock
and follow the instructions on the screen (so obviously it was locked before
and unlocking was successful)

connect to the phoneadb shell

become rootsu

Download the TWRP archive from the linked page on .... and extract the recovery.img).
Next get the recovery.img for TWRP to the phone.

On one of the pages I found on the net they useadb push recovery.img /destination-folder/
but for me this did not work, because when becoming root with su I could not read the contents of the folders where I could push the file to.

So the quick and easy way for me was to download the recovery.img from a server
after becoming rootwget http://my.server/recovery.img
This way I had the file in a directory that I could access.

Then write the recovery.img directly to the necessary device (you are root, you can write anywhere, but you should know what you are doing!)dd if=recovery.img of=/dev/recovery
If that is successful you just made it. Now you have installed TWRP.

reboot recovery

If everything worked you should get into the TWRP recovery you just installed.

Now you can flash the ROM that you have copied to your SD-card, wipe all necessary memories/caches and enjoy your new ROM.

Doogee DG310 Voyager2

Rooting works with baidu root, version 2.4.6 (at the moment available only in
Chinese). I tried framaroot, towelroot, rootgenius and others, none of them worked.
The root is permanent and stays after a reboot.
After rooting install SuperSU instead of the Chinese root Permission program.
Then you can remove all the Chinese programs (one of them displays Chinese ads
in the status bar).

After a permanently crashing google play store I did a factory reset.
The result was a boot loop. So I decided to flash something more modern than android 5 and downloaded resurrection remix OS (7.1.2) from needrom.com.
I flashed a recent TWRP recovery and the boot.img coming in the zip-file.
Unfortunately there was no system.img.
Therefore I decided to format the system/ partition and copy the files by hand.
Boot your phone into recovery and format the system partition.
Connect your phone to the PC.
On your PC change into the $DIRECTORY/system where you downloaded the zip-file.

run
chmod 755 bin/* xbin/*
to set the executable-bit for files that should be executable

for i in `find . -type d`; do adb shell mkdir /system/$i; done
to create all directories in /system/ on your phone

for i in `find . -type d`; do adb push -a $i/* /system/$i; done
to copy all the files to your phone